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NO!! Absolutely not. A pet peeve of mine. Sidewalks introduce massive grit contamination and unless it is a trash stone used for trash knives, don't do it. Some sidewalks are worse than others.

The 24 grit stone will do double duty as a flattener for other coarse grit stones - 60 120 etc.So how does one flatten the flattener? A good way to do it is to rub two 24 grit stones together. The very best way to do it is with three stones. This what serious sharpeners did before diamond plates were around and it still works. It is a technique used for even finer stones, but as you go finer and the fit gets better, you can get some serious stiction issues. For less coarse stones silicon carbide grit on glass or a metal platen is OK. A softer metal is preferred. This is what Shapton had before the diamond lapping plate and it is still used for coarser stones below 1k which the Shapton diamond plates are specifically not used for. You can also put loose silicon carbide on the surface of a XXC stone for flattening other stones. This is the technique suggested by Naniwa for their lapping stone. The 24 Grit Nubatama or Aratae is one tough stone so don't expect that this will need to be done often. In the process of flattening other stones, this helps keep the Aratae stone itself flat.

Rotating 3 stones - a rubs b, b rubs c, c rubs a. This keeps any two stones from fitting flush with each other but still not flat - matching hi and lo spots.

Now I wouldn't plan on going out and getting 3 Aratae stones unless you are doing one hell of a lot of stone flattening. For 120 grit and finer, an Atoma 140 will do the job just fine.

I have a 60 grit diamond plate but this is a somewhat expensive option, but if you are serious about doing a LOT of flattening, it is useful.

For the Aratae, do make a point of using the whole stone for flattening to keep it level as long as possible.

Why stop there? Try flattening a crappy stone using this technique (just kidding of course): I wouldn't DREAM of crapping up a good 150 grit stone with sidewalk debris, especially when an Atoma or DMT XXC will do the job so well.

I can tell you from personal experience, the grit contamination at that level is negligable. You can SEE the particles easily at 150x and lower, so if it's got stuff stuck in it that is bigger than that(which it won't), just rinse it off with a brush. If a brush won't take it out, then return the stone, because you are going to be abrading knife steels on it, and that is going to be way worse for loading up a stone than a sidewalk. Concrete is not lower grit than 150x, and if it were, I'd be sharpening on it. What you are doing is really just abrading down the cement over a large area with your coarse stone, but the sidewalk(or whatever) is so big, you end up wearing the stone down evenly without much technique.

Just to belabor the point, this is not some newfangled or outlandish idea--it is how you polish a concrete floor. There are machines that have little segments on pads that abrade the floor starting at 16 grit or so and shine it up until it looks like it's covered in wax. All you are doing is using the stone to polish the sidewalk, but you rub it on a large enough area that the general flatness of the sidewalk wears down the high spots on your stone.http://www.concretenetwork.com/concrete ... evels.htmlhttp://www.icpsc365.com/industry-inform ... d-segment/All they suggest for glazed abrasives is adding water--that's right, this polishing is done DRY. Your stones, which are both used with water and have MUCH softer binders than these polishing segments, will not have this problem.

I discovered this discipline myself because I flattened a piece of concrete with the 120 grit stone and realized that I'd just worn down the concrete to 120 grit finish and it was pretty smooth. So I put my 500 grit on it. Then my 1k, then my 2k. All the stones were perfectly fine, flat and fresh looking, and that ledge has a very smooth spot on it to this day.

Concrete floors are typically polished with diamond abrasive disks - wet or dry. You would be better off using these disks to polish your stone than the floor. Don't forget that your stone is an abrasive stone whereas the concrete contains a mix of particles of various sizes, hardnesses etc etc that would lend an inferior heterogeneous quality to your stone's homogeneous finish.The object of the exercise is a flat stone, not a shiny floor. It's not that I've never heard of people doing this, just that I feel it is an inferior solution. But to each their own.

Mr. Ken, naive question: my friend uses similar grid stones, he just turns the stones and uses both sides not to duplicate the wearing pattern, is is a good idea? My primary usage is going to be regrinding plain blades and chisels (A2 steel on both). Are Nubatama stones good for this purpose?

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